Nitrogen oxides (NOx), which can have an impact on the environment, are included in exhaust gas emitted from internal combustion engines such as diesel engines. Conventionally, as one aspect of exhaust purification apparatus used for purifying these NOx, there are known SCR (Selective Catalytic Reduction) systems where a selective reduction catalyst is disposed in an exhaust passage and where reduction and purification of NOx is performed using ammonia in this selective reduction catalyst. These SCR systems deliver a reducing agent such as an urea solution capable of generating ammonia to the exhaust passage on the upstream side of the selective reduction catalyst, adsorb the generated ammonia in the selective reduction catalyst, and selectively reduce and purify NOx in the exhaust gas flowing into the selective reduction catalyst.
The saturated adsorption quantity of ammonia in the selective reduction catalyst used in these SCR systems has the characteristic that it changes depending on the catalyst temperature. Further, this selective reduction catalyst has the characteristic that the higher the actual adsorption rate of ammonia with respect to the saturated adsorption quantity of ammonia is, the higher the NOx reduction efficiency becomes. For that reason, control of the dosage of the reducing agent such as the urea solution is performed such that ammonia does not flow out to the downstream side of the selective reduction catalyst and such that the actual adsorption rate with respect to the saturated adsorption quantity becomes as high as possible.
However, sometimes some of the generated ammonia flows out to the downstream side of the selective reduction catalyst due to error in the dosage of the reducing agent instructed by a control unit or degradation of the SCR system over time. The toxicity of ammonia is higher than that of NOx, and a situation where the ammonia is released into the atmosphere without being used for the reduction reaction should be avoided as much as possible. For that reason, there is an SCR system equipped with an oxidation catalyst on the downstream side of the selective reduction catalyst, and when some of the ammonia flows out to the downstream side of the selective reduction catalyst, the oxidation catalyst oxidizes that ammonia, breaks it down into nitrogen gas (N2) and water (H2O), and releases the nitrogen gas and water.
But sometimes the efficiency of the oxidation catalyst drops because of faults including thermal degradation, degradation over time, and cracks. When a fault arises in the oxidation catalyst, the oxidation catalyst cannot sufficiently oxidize and break down the ammonia flowing out to the downstream side of the selective reduction catalyst, and there is the potential for the ammonia to be released into the atmosphere.
Thus, there has been proposed an exhaust purification apparatus configured such that it can perform degradation determination of the oxidation catalyst. Specifically, there has been disclosed an exhaust purification apparatus equipped with an oxidation catalyst that is disposed on the downstream side of a reduction catalyst and oxidizes ammonia in exhaust, second concentration detecting means that detects the concentration of ammonia in exhaust downstream of the oxidation catalyst, and second concentration estimating means that estimates the concentration of ammonia in exhaust downstream of the oxidation catalyst, and when the difference between the concentration of ammonia detected by the second concentration detecting means and the concentration of ammonia estimated by the second concentration estimating means becomes equal to or greater than a second predetermined value, the exhaust purification apparatus determines that the oxidation catalyst has degraded (see patent document 1).    Patent Document 1: JP-A-2006-125323 (claims 4, [0022] to [0027])